1. Field of the Invention
The present invention relates to an image processing method and an image output system. More specifically, the present invention relates to an image processing method, an image processing apparatus, an image processing method for an image output system, and an image output system all of which accomplish high quality prints by preventing the degradation of the image quality on the basis of the combination of densities and sizes of dots used to form images.
2. Description of the Related Art
Many color ink jet printers as color output apparatuses express images using three colors including cyan (C), magenta (M), and yellow (Y) or four colors including these three colors and black (K). In recent years, in order to further improve image quality, large-and-small-dot ink systems and dark-and-light-color-dot ink systems have been proposed; the large-and-small-dot ink system uses different per-droplet amounts for ink of the same color, and the dark-and-light-color-dot ink system has a plurality of inks of similar shades but different densities.
Color materials such as dyes and pigments which are selected for the inks of saturated cyan, magenta, and yellow, primary colors essential to printing, are selected on the basis of ejection characteristics, safety, and the easiness with which they are manufactured. These inks do not always have the ideal hues of the saturated colors. However, it may be desirable that as colors reproduced on a print sheet be faithful to, for example, colors displayed on a monitor.
FIG. 1 is a plot in which CIELab coordinate axes indicate data obtained by executing a calorimetric process on a patch printed with a C, M, and Y inks and values for the colors C, M, and Y on a monitor which values are calculated using an sRGB equation. The figure indicates that compared to the colors reproduced on the monitor, the ink reproduced colors shown in FIG. 1 are such that the cyan ink has a hue closer to magenta, the magenta ink has a hue closer to yellow, and the yellow ink has a hue closer to magenta. To faithfully reproduce the colors Y, M, and C displayed on the monitor, a small amount of C ink may be added to the Y or M ink when the latter is outputted. On the other hand, a small amount of Y ink may be added to the C ink when the latter is outputted. That is, color mixture may be carried out. It is thus easier to make colors reproduced by the printer closer to those reproduced on the monitor by using the ink colors C, M, and Y, which are essential to color printing, to carry out a color conversion such that printed colors in input signal which represent C, M, and Y respectively are closer to the colors C, M, and Y displayed on the monitor.
The above conventional technique is suitable for images such as photographs which contain many nonuniform and complicated colors. However, in the case of images such as graphs which preferably contain uniform colors, in an area of the image which otherwise contains a single color, dots of other colors may appear. These dots may appear granular and disagreeable to users.